Unprecedented warming in the North is altering vegetation productivity and threatening significant soil carbon stocks in peatland and tundra ecosystems. Land surface models (LSMs) are important tools for understanding and quantifying this change; however, high-latitude carbon cycling remains a large source of uncertainty. Accurately representing high-latitude vegetation, such as sedges, and their photosynthetic capacity (e.g. the maximum rate of carboxylation – vmax) is key, but trait variability and data limitations make it challenging. This study quantifies and explores the uncertainty of carbon stocks and fluxes simulated using the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC) associated with sedge vmax variability for a sedge-dominated peatland site in Canada’s Northwest Territories. Field measurements from the TRY plant trait database are used to describe the variability of sedge vmax. To assess the influence of this variability and the associated parameter uncertainty, the observed vmax distribution is systematically sampled to conduct a sensitivity analysis. Finally, simulated carbon stocks and fluxes are compared to observation-based data from an eddy covariance flux tower to further explore and contextualize the results. Sedge vmax variability is found to be a large source of uncertainty in CLASSIC’s simulation of high-latitude carbon cycling. This uncertainty is reinforced by the discrepancy between sedge vmax measurements and CLASSIC’s default vmax value. Comparing sedge vmax simulations to observation-based data suggests that other sedge-specific photosynthetic parameters require examination. Ultimately, this study highlights the need to refine sedge parameterization using trait data to improve the representation of high-latitude ecosystems in CLASSIC.